This invention relates to semiconductor lasers and, more particularly, to high power semiconductor lasers providing high power output in a single diffraction-limited far field lobe.
Many laser applications require high optical power with spatial coherency. Conventional lasers, i.e., solid-state, gas, and dye lasers, can provide these attributes, but the devices are generally large and complex. There is an increasing need to provide compact, electrically-driven semiconductor lasers in such fields as free-space communications, data storage, frequency doublers, and medical applications.
The most common semiconductor laser structure presently used is the quantum well, graded-index separate confinement heterostructure (QW-GRINSCH). This structure contains a thin (&lt;400 .ANG.) small bandgap quantum well gain layer that is bounded by large bandgap cladding materials that are doped n-type and p-type on opposite sides to form an electrical junction. To reduce bandgap discontinuities and provide a better optical overlap to the gain region, the cladding layers are graded from the high bandgap materials to a lower bandgap alloy in the vicinity of the quantum well. Electrical injection of electrons is provided by a stripe of metal on top of the p-type cladding layer and a large area metal contact on the bottom of the n-type substrate layer. If a forward electrical bias is applied to the stripe, photons are emitted from the gain region as a result of electron-hole recombination. A pair of parallel facet mirrors are cleaved perpendicular to the plane of the junction forming what is known as a Fabry-Perot cavity, which provides the optical feedback to the photons emitted from the gain region. At a certain threshold bias, stimulated emission occurs, whereby the feedback provides an emission of light from the junction.
To achieve high power in semiconductor lasers, the volume where gain occurs must be maximized. Due to quantization of the density of states, quantum well structures have higher gain per injected carrier than standard thick (bulk) active layers. To increase the gain from the quantum well structure, it is necessary to increase the gain volume while maintaining the thin transverse dimension of the quantum well. This might be accomplished by using large stripe widths. However, as stripe widths are increased above about 5 microns, filamentation effects and higher-order lateral modes are supported and the laser output can no longer be focused to a small spot in the far field.
The use of narrow stripes with high injection current densities is not viable either; the higher injected current densities result in reduced device lifetimes. Further, the optical power impinging on the facets is limited to the catastrophic optical damage threshold of about 10 MW/cm.sup.2 per facet, which limits the total power obtainable for a given metallization stripe width. Hence, there is a tradeoff with standard Fabry-Perot resonator designs between either having a wide stripe laser having a high output power, but poor coherence, or a narrow stripe laser with good coherence, but limited output power.
To overcome these limitations, many techniques have been examined to achieve high power densities in a single optical mode, including arrays of coupled narrow stripe lasers, master-oscillator power amplifiers, and various unstable resonator geometries. In an unstable resonator laser, the cavity is fabricated such that any light rays that are not directly on the center of the lateral axis tend to diverge out of the lateral boundaries of the cavity. This geometry provides more gain to the fundamental mode, which has its peak energy at the central axis, while higher order modes, which have peak energies off the central axis, experience less gain before they diverge out of the cavity. The net result is that the cavity discriminates in favor of the fundamental mode and the cavity resonates in a single fundamental mode at higher power levels.
In one approach to unstable resonator design for semiconductor lasers, shown in U.S. Pat. No. 5,179,568, issued Jan. 12, 1993, the Fabry-Perot cavity was formed with curved facets for the end mirrors. However, a high precision manufacturing process, such as ion beam milling, is required to maintain losses at the mirrors within acceptable limits. The required high precision does not permit such devices to be formed in quantity and with reasonable reproducibility.
Yet another approach is described in Paxton et al., "Semiconductor Laser with Regrown-Lens-Train Unstable Resonator: Theory and Design," 29 IEEE J. Quantum Electron., No. 11, pp. 2784-2792 (November 1993). A train of weak negative cylindrical lenses is grown into the structure to cause the fundamental mode to expand laterally as it propagates. For large changes in the effective index of refraction, however, each lens will act as a reflecting surface (as well as a diverging element) and will introduce concomitant cavity losses.
One other approach, described in Chan et al., "Antiguiding Index Profiles in Broad Stripe Semiconductor Lasers for High-Power Single-Mode Operation," 24 IEEE J. Quantum Electron., pp. 489-495 (1988), provides a continuous variation of the index of refraction in the lateral dimension, with increasing index of refraction from the center of the resonator, so that the rays of the resonator mode curve away from the center of the laser. Simple, cleaved planar end mirrors may be used. But the device taught by Chan et al. provides an antiguiding layer of Ga.sub.0.7 Al.sub.0.3 As regrown on Ga.sub.0.85 Al.sub.0.15 As. These structures oxidize during the regrowth process with resultant poor electrical and optical performance (see, e.g., G. Guel et al., 21 J. Electron Mat. 1051 (1992)).
Thus, the unstable resonator approach provides a means of obtaining a high power output with spatial coherency. In accordance with the present invention, a standard Fabry-Perot cavity is provided for laser resonance and a continuous, diverging medium is formed in the semiconductor structure, without significant oxidation effects, to cause higher order lateral modes to diverge out of the pumped region.
Accordingly, it is an object of the present invention to provide a semiconductor laser with a standard Fabry-Perot cavity for optical feedback and a continuous internal structure for removing unwanted higher order lateral lasing modes.
It is also an object of the present invention to provide an antiguide layer for filtering out high order modes while enclosing the antiguide layer in cladding that is not subject to oxidation that cannot be removed.
Another object of the present invention is to provide a semiconductor laser with reduced scattering and reflection losses.
Yet another object of the present invention is to remove unwanted higher order modes from the resonant cavity before amplification of the unwanted modes.
One other object of the present invention is to provide an unstable resonator that is easily manufactured with reproducible characteristics.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.